The present invention concerns a hand greas gun with a lever-operated piston pump and a tubular grease supply reservoir. More specifically, the invention relates to a portable device for pressure delivery of a highly viscous lubricating material, which device substantially consists of a cylindrical container with a supply of the lubricating material, and a reciprocating delivery pump, forming part of the head of the device. The reciprocating pump is manually operated through a lever arm, and is connected with its suction port to the container and with its delivery port to an end dispensing nozzle. Such kinds of devices are also known as syringe grease guns.
As it is known, in the manual grease guns of the kind mentioned above the high viscosity of the material involved makes it necessary to combine the action of the piston pump on the head of the device with a further thrust, which helps the grease mass to flow towards the delivery end of the container, by pressing the mass from the rear end of the container. For this reason, in the most widespread grease guns, either of the type refillable with grease in bulk or of the type for use with commercial grease cartridges, there is provided a back pressure piston, slidable within the tubular container. The pressure piston pushes the grease mass towards the head piece, moving gradually forward as the grease supply diminishes.
In the conventional syringe grease guns, the pressure piston is biased by a helical compression spring, which is housed in the rear region of the cylindrical reservoir. The compression spring is loaded, before filling the container with grease, by pulling a suitable draw rod coaxial to the spring. One end of the draw rod engages the pressure piston at the center thereof, while the distal end of the rod is provided with a hook or a handle, and extends out of the rear end of the cylindrical container. Specifically, to load the grease gun, the head assembly with pump and lever arm is first removed from the device, by unscrewing the screw cap and seal through which the assembly is connected to the front end of the tubular container. Then, the draw rod handle is grasped and the pressure piston is retracted against the bias of the spring, which is thus compressed against the rear inner wall of the container. Normally, the draw rod is provided with a locking element which cooperates with a mating element fixed on the rear region of the tubular container, so that the draw rod may be blocked in its fully retracted position, almost completely extending out of the reservoir, which corresponds to the position of maximum compression of the spring and to that of maximum retraction of the pressure piston within the cylindrical body of the container.
While the pressure piston is being kept in this position, the tubular reservoir is filled with grease in bulk, by manually introducing it from the open front end of the cylinder. In the alternative, the tubular reservoir is loaded with a grease cartridge, which is also inserted from the front end of the cylinder, after having removed both end covers of the cartridge. After closing the device by again screwing the head assembly thereon, the draw rod may be released from its locked position, thus releasing the spring, and the latter may thus exert its pressure, through the pressure piston, on the grease mass in the reservoir. It is clear that, in the simplest case where the draw rod is integral with the pressure piston, the rod protrudes from the rear end of the grease gun a length corresponding to the degree of loading of the tubular container, and is gradually retracted within the container as the grease supply decreases.
During operation, the spring and pressure piston assembly pushes the grease towards the front end of the tubular container. A small aperture is provided on the front end, communicating with the intake port of the reciprocating pump. When the lever of the pump assembly is raised, the pump piston is caused to move backwards in its cylindrical chamber, thus opening the suction port of the pump. This results in a small amount of grease being sucked into the chamber. When the lever of the pump is lowered back, the pump piston pushes this small amount of grease beyond the delivery port of the pump, which is normally closed by a globe check valve. The grease is thus ejected from a nozzle provided on the head of the grease gun, the said nozzle being normally screw-connected to a small flexible hose for dispensing grease in the desired points of the mechanical apparatus receiving the lubricant. The grease transfer into the pump chamber is due to the suction pressure caused by the piston stroke, but it is strongly aided by the pressure exerted by the pressure piston on the whole mass of grease, towards the head of the device.
A manual grease gun of the kind described above, preferably of the type for use with cartridges, is disclosed, e.g., by U.S. Pat. No. 4,298,144. The patent relates to some minor modifications of the conventional device, such as the shape of the locking arrangement for blocking the draw rod in a retracted position, and the provision of a narrow air outlet opening through the screw cover of the head piece, to allow the expulsion of any air entrapped in the grease. In order to reduce the inconvenience caused by the draw rod, in the grease gun illustrated this said document the draw rod is not fixedly connected to the pressure piston, but is slidable within a bore provided on the piston, coaxial thereto. The end restraint is provided by a nut screwed at the end of the draw rod. As a result, when the rod handle is pulled backwards the draw rod entrains the pressure piston with it, thereby causing the spring compression, but, once the container has been filled in and the draw rod is released from its locked position, the draw rod may be reinserted back into the grease mass contained in the reservoir, by sliding with respect to the pressure piston. The latter, on the contrary, is blocked in its totally retracted position by the grease mass filling the container.
Substantially the same type of conventional grease gun with spring-biased pressure piston is disclosed in EP-A-0 379 681. This document concerns the provision, in the grease gun head piece, of a second valve connected to the piston pump chamber. This second valve is endowed with an overpressure indicator, for monitoring any abnormal pressure peaks in the grease gun.
In order to allow the use of cartridges with collapsible containers, which are opened by taking away one cover only, from the end which in use will face the grease gun head, EP-A-0 528 270 proposes a syringe grease gun still of the spring-biased type, but having no draw rod. (As a matter of fact, the draw rod, in the version where it is slidable with respect to the pressure piston, would interfere with the rear wall of the collapsible container.) Instead of the rod there is provided, as a means for pulling the pressure piston towards the back end of the container against the bias of the spring, a flexible cord that is automatically rewind within the grease gun, on a winding spool connected to a spiral spring. The rewinding device is housed within the container, immediately behind the pressure piston, in the region enclosed within the coils of the helical compression spring.
The three prior art solutions referred to above, applied to conventional grease guns with spring-biased pressure pistons, achieve an appreciable reduction of the original hindrance caused by the draw rod in the fully loaded condition of the grease gun. However, these solutions are not free from a number of drawbacks typical of conventional grease guns, essentially due to the use of a mechanical spring means as a source of pressure for pushing the grease of the cylindrical reservoir towards the head piece. As a matter of fact, it is known that the force provided by a spring is proportional to its elongation (or to its shortening, in the case of a compression spring) with respect to its length at rest; therefore, the force that the compression spring may exert decreases as the spring expands, and the maximum pressure is exerted only when the tubular container is totally full. As a consequence, specially at low temperature, when the grease is particularly viscous, the delivery of the last portion of grease contained in the grease gun becomes particularly difficult, since the pressure piston unsatisfactorily cooperates with the piston pump of the head. As the spring is compressed by pulling the draw rod manually, it is evident that the stiffness of the spring cannot be increased too much, not only because the loading would become too difficult, if not impossible, but also because an accidental release of the draw rod locking system during the loading operation would be more and more dangerous. In the current practice, the maximum strength provided by a totally compressed grease gun spring does not exceed 15-20 kg. If one considers that the concerned devices are to be used on the spot, most of the times in the open air and often in very cold weather conditions, it is readily understood that the spring and pressure piston system may cause an unsatisfactory working of the grease gun. Thus, the complete use of the grease supply contained in the tubular reservoir is sometimes prevented.
The above problem becomes more critical if the piston pump is slightly worn out, or if the seals thereof do not perfectly work. In this case, the suction that the piston pump may exert decreases, as air penetrates into the pump chamber during the suction stroke. As a consequence, the amount of grease delivered at each stroke of the lever progressively decreases, and the number of pumping strokes are to be increased accordingly, until the pump finally stops working.
It is also to be taken into account that the spring and pressure piston system, besides being hardly adaptable to any thermal variations in the working conditions, is also scarcely adaptable to any changes in the quality of the grease employed. As a matter of fact, even in favorable weather conditions a grease gun which works satisfactorily with a certain type of lubricating product may turn to work unsuitably if the product is replaced with a more viscous one.
A kind of syringe grease gun wherein the rear pressure piston is not operated by a spring is disclosed by EP-A-0 228 991. This document relates to a device similar to those discussed in the foregoing, but wherein the spring is replaced by an air-tight chamber loaded with compressed air. The chamber is provided in the rear section of the cylindrical container. Compressed air (or any other suitable gas) is introduced in the chamber only once, before the use outdoors, by applying a compressor or a compressed air bottle to a valve provided on the rear end of the grease gun. Between the air-tight chamber and the grease mass contained in the grease gun there is provided a piston of the double-acting type, wherein both sides of the piston are active, and thus at least two annular seals are present. As a matter of fact, the piston must not only push the grease towards the head of the grease gun, but it must also be impervious to the compressed air which presses on the other side thereof. The compressed air is not supplemented during operation, but it must remain permanently entrapped in the air-tight chamber, and therefore the grease gun is not opened by unscrewing its head assembly for refilling it. For this purpose, a grease inlet nozzle is provided on the grease gun head, through which grease is injected in the tubular container by means of a suitable pump.
The device apparently achieves a remarkable reduction of the grease gun volume, as well as a reduction of its production costs, thanks to the elimination of the spring and draw rod assembly. In addition, the device is somehow more adaptable, since the compression supplied to the pressure piston may be adjusted by increasing or by reducing the pressure in the air-tight chamber. In order to obtain this result it is not required, for instance, to change any mechanical component such as a spring. However, since the compressed air supply cannot be supplemented (unless a proper compressor, or at least a compressed air bottle are available on the spot), at each working cycle the pressure exerted on the pressure piston decreases as the latter moves towards the head section, exactly as it happens when a spring is used. Also in this case, the pressure in the air-tight chamber cannot be excessively raised, at least because this would result in rendering prohibitive the refilling operation by pumping grease through the front nozzle.
In addition, the above grease gun is not an autonomous device, but requires on one hand a source of compressed air and, on the other hand, auxiliary equipment (in particular, according to the above patent document, a manually operated grease pump) for refilling it with grease. It is clear, therefore, that this device does not help in satisfactorily overcoming the above mentioned problem of the difficult delivery of grease in critical working conditions. Moreover, the device appears to bring in further practical complications with respect to the refilling of the tubular container. Finally, it is to be noted that the hand gun disclosed can be employed with grease in bulk only, and is not suitable for use with cartridges.
Another hand grease gun wherein the rear pressure piston is operated by compressed air introduced in an air-tight chamber provided in the back section of the cylindrical reservoir is disclosed in U.S. Pat. No. 2,463,113. In this case the compressed air needed for biasing the pressure piston against the grease mass in the reservoir is not provided by an external source, rather it is supplied by a manual reciprocating air compressor mounted on the grease gun. The delivery port of the air compressor is connected to the back pressure chamber of the cylindrical reservoir by means of a flexible hose extending through the whole length of the reservoir, as the air compressor is located close to the head piece of the grease gun. This position of the reciprocating air compressor is due to the fact that the latter is operated from the same lever arm which operates the grease pump. Thus, the oscillating air compressor is pivoted with one end on a fixed support close to the grease gun head, and with the other end (connected with the piston element of the air compressor) on a bracket provided along the lever arm of the grease gun. Accordingly, each oscillation of the lever arm corresponds both to a stroke of the grease piston pump and to a stroke of the reciprocating air compressor.
As a consequence of the above connection of the air compressor to the same lever arm which operates the grease pump, it is not possible to put the rear chamber of the tubular reservoir under pressure independently of the grease pumping process. Thus, at the start of a greasing operation the amount of air behind the pressure piston is insufficient to efficiently prime the grease gun, and the first strokes will unavoidably deliver a reduced amount of grease. Subsequently, as the pumping proceeds, the pressure in the rear chamber continuously and indefinitely increases (the amount of grease pumped per stroke increasing accordingly), and the manual work required at each stroke also increases, without any possibility of disconnecting the air pumping device.
Furthermore, since the air compressor cannot be operated independently of the grease pump, the disclosed device cannot be adapted to variations in the viscosity of the grease treated (either due to changes in the external temperature or to the use of different grades of grease), as the air pressure supplied in the rear chamber cannot be adjusted as desired.